Hydrothermal growth of rare earth orthoferrites and materials so produced



Dec. 23, 196-9 E. D. KOLB ETAL 3,485,759

HYDROTHERMAL GROWTH OF RARE EARTH ORTHOFERRITES AND MATERIALS S0PRODUCED Filed Aug. 15, 1967 FIG FIG 3 22a 23 24a 22b 236 246 DOM/UNSENS/N6 NUCLEAT/NG MEANS SOURCE E. 0. KOLB VENTORS" 2 2 23 fificD.L.WOOD

HYDROTHERMAL GROWTH OF RARE EARTH ORTHOFERRITES AND MATERIALS S PRODUCEDErnest D. Kolb, New Providence, and Robert A. Laudise and Edward G.Spencer, Berkeley Heights, and Darwin L. Wood, Murray Hill, N.J.,assignors to Bell Telephone Laboratories, Incorporated, Murray Hill andBerkeley Heights, N.J., a corporation of New York Filed Aug. 15, 1967,Ser. No. 660,643 Int. Cl. ClOm 35/69, 35/40 U.S. Cl. 252-6257 6 ClaimsABSTRACT OF THE DISCLOSURE Rare earth orthoferrites includingcompositions containing mixed rare earths are grown hydrothermally froma potassium hydroxide solution. Resulting crystals have excellentelectrical and magnetic properties which are comparable with the bestwhich have been measured on flux-grown materials of the samecompositions.

BACKGROUND OF THE INVENTION Field of the invention The invention isconcerned with the preparation of single crystals of rare earthorthoferrites. Such crystals are of interest in a large class of devicesincluding those which depend for their operation on the nucleation andpropagation of single wall magnetic domains of restrictedcross-sectional area.

Description of the prior art SUMMARY OF THE INVENTION In accordance withthe invention, it is found that sound single crystals of both single andmixed rare earth and yttrium orthoferrites may be hydrothermally grownon an immersed seed using concentrated aqueous potassium hydroxidesolution as the transfer medium. Magnetic and electrical properties aregenerally comparable to those of the flux-grown crystals. Crystals ofsingle and mixed rare earth orthoferrite compositions grown by thistechnique are found to be physically sound and to evidence the requisitecrystalline perfection for device use over cross-sectional areas of theorder of one square centimeter and larger. A generalized formula ofcompositions grown in accordance with the inventive process may beexpressed as follows:

RFCOg where:

R is one or more of the elements of atomic numbers 39 and 62 through 70of the Periodic Table of Elements (that is, yttrium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium)and which may additionally contain from 0 to about 20 atom percent ofelements numbers 57 through 60 (that is, from 0 to 0.2 in the formula oflanthanum, cerium, praseodymium or neodymium).

Generally, at least 80 atom percent of the R atoms should be selectedfrom elements 39 and 62 through 70,

nited States Patent O as indicated, for the reason that growth of largecrystals of orthoferrites becomes difficult for R atom ionic radiilarger than that of samarium. It is known that lanthanum tends to go +4and that europium tends to go +2 during orthoferrite growth. Retentionof the trivalent state may be assured by the presence of hydrogen orother reducing agent and oxygen or other oxidizing agent, respectively.

The purpose for such cation variations has to do with deviceconsiderations, :1 complete discussion of which is not consideredproperly included in this description. Basically, however, a major classof device uses contemplated requires the nucleation and propagation ofsingle wall domains. Desired bit density and other engineeringconsiderations give rise to a requirement not only of domain stabilityand other obvious desiderata such as ease of nucleation, etc., but alsoto a desired domain size. It has been found that domain size, as well ascertain other characteristics, depends, inter alia, on the proximity ofthe operating temperature to a transition temperature, notably to thespin flop transition temperature. Difierent rare earth inclusions giverise to different spin flop temperatures and consequently to a differentrange of domain sizes for given operating temperatures. Of theenumerated cations, only samarium gives rise to an orthoferritecomposition having a spin flop temperature above room temperature. Forcertain types of operation, it is desired to operate single wall domaindevices near the spin flop temperature. For such purposes samiriurn maybe combined with one or more other cations to produce materials having atailored spin flop temperature near any operating temperature.

Magnetically, the rare earth orthoferrites are canted antiferromagnetic.The magnetization and certain other magnetic properties depend on thecanting angle and also on the degree to which the material is completelyantiferromagnetic, that is, the extent to which the opposing moments areequal. Partial substitutions for iron, for example, with gallium oraluminum may result in a variation of the saturation magnetization, orin alteration of other magnetic properties. Device significance mayfollow, for example, from the fact that increased magnetization resultsin a decrease in stable domain size. Certain specific complete andpartial substitutions have been outlined. Other variations, both in therare earth and in the iron sites are feasible. Any such substitutionsnecessarily produce a concomitant change in magnetic properties. Allsubstituted materials which retain the rare earth orthoferrite structureare desirably grown in accordance with the process of the invention and,in consequ nce, are considered within the inventive scope.

Crystals resulting from use of the inventive procedures are suitablyincorporated in devices depending for their operation on the electrical,magnetic, or acoustic properties of these materials. Such crystals anddevices form a part of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view, partly incross-section, of apparatus suitable for practice of the inventiveprocesses;

FIG. 2 is a perspective view of an orthoferrite crystal grown inaccordance with a process herein; and

FIG. 3 is a schematic representation of a device depending for itsoperation upon the nucleation and propagation of single wall domains ina material of the invention.

DETAILED DESCRIPTION Referring again to FIG. 1, there 1s depicted thenow familiar modified Bridgman apparatus used for hydrothermal growth.The only significant difierence from apparatus used for quartz growth isthe inclusion of a precious metal liner of, for example, platinum,desirably incorporated because of the increased reactivity of thesystem. The main body 10 contains a precious metal can 11, so defining achamber 12. A main nut 13 is threaded into the upper portion of thechamber. A plunger 14' is fitted into thechamber 12 and is free to risewith increasing pressure in the chamber. As the plunger rises, itcontacts a steel seal ring 15 and is finally stopped by bearing againstthe main nut 13 through the seal ring. This action provides an effectiveseal for the growth chamber. The chamber is initially temporarily sealedby means of the set screws 16 which compress a resilient washer 19against the shank of the plunger. The space between the can 11 and theinner wall of body 10 is filled with water to a degree necessary tominimize pressure differential between the inside and outside of can 11.

For the growth procedure the chamber 12 is charged with nutrientmaterial. The potassium hydroxide solution is added in the amountrequired to produce the requisite pressure at the desired operatingtemperature. Seed crystals such as 17 are suspended as shown. A baffle18 may be interposed between the nutrient mass and the seed crystals soas to divide the chamber into two thermal zones. The baflie maintains areliable temperature differential between the nutrient and thecrystallization zone and expedites simultaneous growth of two or moreseeds.

Chemically, the nutrient may be such as to yield the desiredorthoferrite composition. T this end the starting materials may be rareearth oxide and iron oxide (Fe O included approximately in thestoichiometric ratio although some deviation of 11p to about percent ispermitted. Alternatively, the nutrient may be prereacted in particulateor bulk form. Reacted nutrient may result from sintering or flux orhydrothermal crystallization. The transfer medium is a water solution ofpotassium hydroxide, the permitted range of concentration being from 10molal to 25 molal. The lower limit is dictated by the facts that (1)other phases appear at appreciably lower concentrations and (2)reduction in growth rate to inexpedient values results from use of lowerconcentrations. If the base concentration is much higher than 25 molal,attack on the noble metal liner becomes a problem, The preferredconcentration range is from 2025 molal, the preferred lower limit beingselected largely on the basis of growth rate.

The fill is desirably from 70 percent to 95 percent by volume althoughthese limits are not absolute. For the permitted temperature range offrom 350 C. to 425 C., the resulting pressures are such that amechanical problem is introduced above 95 percent. The lower fill limitis based solely on growth rate, with rates dropping below a convenientlevel for lower fill. Temperatures are interrelated with rates, so thattemperatures above that indicated result in pressures which are undulyhigh for usual autoclave structures and with growth rate dropping undulybelow the lower indicated temperature. While generally still highertemperatures are permitted for lower fills below the indicated minimum,there are some disadvantages in this procedure in that phases other thanthe orthoferrite tend to form. Still lower temperatures correspondingwith still higher fill percentages do not generally result in acceptablegrowth rates. Pressures corresponding with these temperatures range fromabout 8,000 p.s.i. for 350 C. and 70 percent fill to about 35,000 p.s.i.for 425 C, and 95 percent fill.

As in any hydrothermal growth procedure, a significant parameter is thetemperature difierential between the seed and nutrient. The use ofsmaller differentials reduces the growth rate, but, in common with thechoice of other parameters which minimize the rate, results in greaterperfection due to the fact that more time is permitted for rearrangementof atoms on the surface of the growing crystal. A minimum gradient ofabout 5 C. is specified. This value arises from consideration ofpermissibly small growth rate and from the fact that temperature controlof smaller gradients is generally diflicult with commercially availableapparatus. A preferred minimum of 10 C. is recommended. The maximumtolerable gradient is considered to lie at about 50 C. since forsignificantly larger values spontaneous nucleation becomes a problem. Apreferred maximum lies at about 30 C. The crystal structure of the rareearth orthoferrites is orthorhombic. Preferred seed plate orientation is[110] or [001]. In general, growth-rate is most rapid on the former. Useof an [001] seed, however, is desirable for certain compositions anduses, since it results in an easy magnetization direction orthogonal tothe crystal sheet. Growth rates as high as 6 mils a day have beenobserved for a 20 mole KOH solution, percent fill, 375 C.crystallization temperature, and temperature differential of 30 C.(-8,000 p.s.i.). As in other hydrothermal growth procedures, thetemperature gradient is largely controlled by a baffle such as baffle 18in FIG. 1. A convenient open area for the bafiie is about 5 percent.Much larger than 10 percent open tends to decrease the temperaturegradient to values below that permitted in the usual apparatus.

The following examples describe specific parameters and compositionswhich have been utilized to produce some of the crystals which have beendescribed.

EXAMPLE l Apparatus similar to that depicted in FIG. 1 of approximateinner dimensions 2% inch length by one inch diameter was utilized. 8grams total of Fe O and Yb O in the mo-l ratio of 1:1 were placed in thebottom of the autoclave. The bafile, such as that shown as element 18.was then placed in position. seeds such as 17 were placed in theposition shown. The autoclave was filled to 80 percent of its freevolume, with 20 molal aqueous KOH. The autoclave was closed and wasplaced in a furnace where it was brought to a temperature of 375 C. atthe seed position in a period of about five hours. The bottom of theinner vessel corresponding with the nutrient position was at this pointat a temperature of about 405 C. (a temperature differential of about 30C.). The pressure under these conditions was about 8,000 p.s.i.Autoclave and contents were maintained under these conditions for aperiod of about 30 days, after which the autoclave was removed from thefurnace, was permitted to cool to room temperature in a period of about10 hours, after which it was opened and the seed crystals, together withnew growth, removed. The resulting growth was of the order of 180 milstotal (corresponding with a growth rate of about 6 mils a day).

EXAMPLE 2 The preceding example was repeated as described, however,substituting presintered Fe O +Yb O for the starting materialsindicated. The growth rate was substantially unchanged. The resultingcrystal was about 1 cm. by 1 cm. by 210 mils and was sound andmagnetically homogeneous.

EXAMPLE 3 The procedure of Example 1 was repeated, however substitutinga sintered mass of the composition Sm Er FeO for the nutrient thereinindicated. The final crystal was of the same composition as that of thenutrient. Crystal size was of the order of 1 cm. by 1 cm. by 210 mils.Magnetic properties were of device caliber.

EXAMPLE 4 The procedure of Example 1 was repeated, however substitutinga sintered mass of the composition YFeO for the nutrient of thatexample.

EXAMPLE 5 The procedure of Example 1 was repeated, however substitutinga sintered mass of the composition HoFeO for the nutrient of thatexample.

EXAMPLE 6 The procedure of Example 1 was repeated, however substitutinga sintered mass of the composition TbFeO for the nutrient of thatexample.

Products of Examples 3 through 6 were of the same composition as thestarting material and were sound and magnetically homogeneous.

These examples represent but a small number of those actually carriedout. All of the outlined compositions including single or mixed cationsof elements 39 and 62 through 70, as well as compositions containing upto atom percent of the other elements lanthanum and Nos. 58 through 60,as well as compositions containing other substituents permitted in theorthoferrite phase, are expediently grown without altering the outlinedgrowth conditions.

The rare earth orthoferrites are desirably utilized in a class ofdevices which include a sheet or layer of a single crystalline materialwhich is magnetically isotropic in the plane of the sheet and which hasan easy direction out of the plane of the sheet. An exemplary use is ina shift register. The device of FIG. 3 is described in such terms.

In FIG. 3 a register 20 comprises a sheet 21 of a rare earthorthoferrite in accordance with the invention. The sheet is so orientedthat at the operating temperature the preferred magnetization direction(easy direction) is normal to the plane of the sheet. Flux direction outof the paper as viewed is represented by a plus sign. Flux directed intothe paper is represented by a minus sign. Conductors 22, 23, and 24,which may be deposited on the surface of sheet 21, form triplets ofloops 22a, 23a, 24a; 22b, 23b, 24!), et seq. Loop size is somewhatsmaller than the size of a corresponding stable single wall domain sothat in operation any magnetized domain is partly within an adjoiningloop. Such domains, once nucleated, for example, by means of a domainnucleating source 25 and loop 26, are stepped from loop position 22a to23a to 24a to 22b and so forth by successive energization of conductors22, 23, and 24 in that order by means not shown. Readout is accomplishedby means of loop 27 and sensing means 28.

Other device uses include switches, other types of memory elements,logic elements, etc. Some such devices may operate at constanttemperature at or near the spin flop temperature. Others may depend on atemperature variation sometimes local to flip the magnetization and soprovide a means for easily nucleating a domain.

In other manner, the device description has been rudimentary. Devices ofthe type depicted in FIG. 3 have been developed to a far more advancedstate. Some no longer utilize looped conductor configurations but dependupon the flux concentration which results from a sharp turn in theconductor pattern. A simple zig-zag pattern,

for example, results in a bit location at each conductor reversalposition. More generally, while present interest largely centers on theuse of the materials of this invention in single wall domain elements,other devices may depend upon more conventional properties such, forexample, as overall changes in magnetization, in changes in transmissionproperties of electromagnetic energy, under the influence of an appliedfield or with temperature change, of yielding the said material in analkaline solution within ventive scope.

It is clear from the description that most uses contemplate sheetmaterial. Sheet material has been prepared by growth of a limitedthickness on a seed as indicated in the examples. Subsequent treatmentmay include slicing and, finally, back sputtering to remove damagedportions. Crystals have also been grown epitaxially. Single crystalgrowth, of course, requires a substrate material having lattice'dimensions closely approximating those of the orthoferrite to be grown.For device purposes, it is generally desired that the substrate beessentially nonmagnetic. The paramagnetic material terbium aluminate(TbAlO has been found an appropriate substrate for the epitaxial growthof YbFeO For expediency, the invention has been described in terms of alimited number of embodiments. In general, variations in growthconditions and grown compositions have been set forth. Also, it has beenindicated that the device uses specifically described are merelyexemplary. The invention is considered to reside in the finding that theuse of aqueous KOH solutions in the indicated concentration range in ahydrothermal process results in the growth of large sound orthoferritecrystals.

What is claimed is:

1, Method for growing crystalline material which comprises disposing acrystal and a mass of nutrient capable of yielding the said material inan alkaline solution within a closed vessel, heating said solution to atemperature of at least 300 C. while under a pressure exceeding itscritical pressure and maintaining a temperature difference between saidseed and said mass of nutrient of at least 5 C. until a desired amountof crystalline growth results on said crystal, characterized in that thesaid solution consists essentially of a 10 to 25 molal aqueous solutionof potassium hydroxide and in that the said crystalline materialconsists essentially of the composition RFeO where R is at least oneelement selected from the group consisting of yttrium Samarium,europiurn, gadolinium, terbium, dysprosium, holmium, erbium, thulium andytterbium, additionally containing from O to 20 atom percent of at leastone element of the group consisting of lanthanum, cerium, praseodymiumand neodymium.

2. Method of claim 1 in which the said mass of nutrient comprises asintered mass of the desired composition.

3. Method of claim 1 in which the said mass of nutrient compriseshydrothermally grown material.

4. Method of claim 1 in which the said solution consists essentially ofa 20 to 25 molal solution of potassium hydroxide.

5. Method of claim 1 in which the said vessel is filled to within topercent of its volume before heating.

6. Method of claim 1 in which the said temperature difference is withinthe range of from 10 C. to 30 C References Cited Bertaut et al.Structuredes Ferrites Ferrimagntique des terres rares-Comptes Rendus, vol. 242,pp. 382-4.

Laudise et al.Solid State Physics, pp. 2102l3.

TOBIAS E. LEVOW, Primary Examiner R. D. EDMONDS, Assistant Examiner US.Cl. X.R. 235l

